In the field of exposure apparatuses used in semiconductor production processes, recent years have seen powerful advances in the techniques used to miniaturize and exposure-transfer ever-finer circuitry patterns, with corresponding attempts to shorten the exposure wavelengths used in an effort to further improve resolution.
As exposure wavelengths continue to shorten, KrF and ArF excimer lasers, which are types of gas lasers, have become more common.
These excimer lasers are able to select a specific wavelength using a narrow-band module, and so can achieve generation of specific, extremely narrow-band modulated pulses of light through oscillation development. The specific wavelength is to some extent changeable.
With this type of conventional exposure apparatus, a pattern on a reticule (mask) is exposure-transferred onto a semiconductor wafer by the pulses of light generated by the excimer laser.
FIGS. 2A and 2B are diagrams illustrating a spectral distribution of an excimer laser.
Since the above-described laser is a gas laser, the gas must periodically be changed. However, changing the gas also changes the compositional ratio of the gas inside the chamber, and as a result the spectral distribution of the pulse sometimes also changes as shown in FIG. 2A.
Similarly, the characteristics of the optical components in the above-described narrow-band module sometimes also change over time, causing the spectral distribution to shift as well.
Similarly, the above-described excimer laser, though having the ability to hold the central wavelength constant as described above, nevertheless sometimes experiences a slight shift in central wavelength as shown in FIG. 2B due to a variety of causes internal to the excimer laser.
As described above, due to a variety of factors, the excimer laser used in exposure apparatuses is susceptible to changes in the shape of the spectral distribution and shifts in the central wavelength. If and when such changes and shifts occur, they can negatively affect the exposure apparatus's projective transfer capability.
More specifically, any shift in the width of the spectral distribution changes the contrast of the projective transfer pattern. Since the degree of impact varies depending on the pattern to be transferred, a shift in contrast not only degrades the transfer pattern contrast, but also deforms the transfer pattern itself.
In addition, if the central wavelength fluctuates, then the focus of the exposure apparatus will also change and it will become difficult to transfer fine patterns.